Trouble is, kitchen knives and machetes, axes etc, can be argued to be "tools". You can go buy them down at B&Q if your over 16. The fact that they are potentially very lethal, gets muddied when you consider "what is the purpose of a katana" answer: it is made to chop things, traditionally human flesh.
We therefore have to argue that we keep them in order to practice an art form. This is similar to the "pistol shooting" debate, despite it being an Olympic sport ( at which GB used to do very well) its all gone tits up following Dunblane. (and have we seen a decrease in gun crime since that awful day?)
With the (alleged) increase of attacks with katana (I'd like to see the evidence for this and match it against short knife casualties) the government will jump on the issue, ban or restrict their use or ownership, and then use it to crow about how they are tackling crime involving bladed weapons. Despite the huge number of people signing the on-line petition this bast@rd government will just do what it thinks is popular for itself.
Gonna be a lot of fingers lost in Gwynedd when the "Katana inspector" comes to confiscate....methinks Llanrug may be a particular hotspot of trouble

Lloydie